The UK advertising watchdog has banned a campaign by the retailer American Apparel for using a partially nude model, who appeared to be under 16, in a series of images that suggested she was "stripping off for an amateur-style photo shoot".

The Advertising Standards Authority received a complaint that it was offensive, unsuitable to appear in a magazine that could be seen by children and inappropriate because the model seemed "young and vulnerable and [the ad] could be seen to sexualise a child".

American Apparel said the ad was meant to depict the 23-year-old model in a relaxed "home" environment and that the hoodie shown was "soft to the touch" and could be worn directly against the skin. The company added that the ad focused on the hoodie and did not portray the model as "a sex object or in a negative or derogatory light".

Vice magazine, with a circulation close to 90,000 in the UK, targets 18- to 34-year-old readers who would not be offended by the image, the company said.

The ASA agreed on the general point that the amount of nudity shown in the ad was not in breach of the advertising code, given the carefully targeted use of Vice.

However, the ASA said that in some of the images the model appeared to be under 16 and that they were "provocative with the model exposing progressively more skin in each photo of the series".

"We considered that the photographs suggested that she was stripping off for an amateur-style photo shoot," the ASA said in its ruling banning the ad. "Because the ad could be seen to sexualise a model who appeared to be a child, under the age of 16 years, we concluded that it was inappropriate and could cause serious offence to some readers."

"American Apparel is well known for its provocative advertisements," said the American Apparel UK operations manager, Brent Chase, in a statement on the ASA's ruling.

"Our models are real girls who are often employees or friends of the company. They do their own hair and makeup and aren't Photoshopped. From time to time people are made uncomfortable by this, and it occasionally causes an unfortunate reaction."

It is not the first time the US clothing chain has been in hot water this year over its advertising tactics.